From what the OP is saying, the tuner OFFERED to do it, as a service, regardless of whether it was payable for or not, while working on his car, which would all be included for in the price of the tune and all anyway.
If the OP had personally asked the tuner to blow oxy at it, then it would be a different story. Because the OP trusted the tuners work, he thought he would let him do what he thought best for his car.
Its very easy for the tuner to say its NOT his fault. It might have been over torqued unintentionally, esp when it was hot. If the tuner offered to blow the plugs free of charge, then he should have installed them as well. Thats pretty careless to me, considering he is meant to be the professional here. After all he was relying on the OP to install the plugs CORRECTLY, before ramping it up to full load on the dyno.
Im not sure whether you guys have the Fair trading act over there, but I would put him down for Neglegence and serious mis-conduct. OP has evidence as well - ceramic bits, right? I would say the tuner is fully responsible, and should cop up AT LEAST 80% if not 100% of the cost to get this fixed.